Photoconductive cell



N. C. ANDERSON PHOTOCONDUCTIVE CELL Filed Nov. 1'7, 1958 FIG. 1 FIG. 2

FIG. 3

3w g xv AMPLIFIER 23 31 FIG. 4

INVENTOR N.C. ANDERSON ATTORNEY A b 3 n u United States Patent Ofiice 3,076,949 Patented Feb. 5, 1963 3,076,949 PHOTOCONDUCTIVE CELL Norman C. Anderson, Wellesley, Mass., assignor to Infrared Industries, Inc., Needham Heights, Mass., a corporationof Delaware Filed Nov. 17, 1958, Ser. No. 774,197 2 Claims. (Cl. 338-18) This invention relates to photoconductive cells.

There are applications in which photoconductive cells are used in searching and tracking a source of radiation. For instance, some anti-aircraft missiles search for and track enemy bombers, the engines of which are excellent sources of infrared radiation. Usually a photoconductive cell having a large area is used to do the searching and, once the source of radiation is located, a photoconductive cell of much smaller area is used to do the tracking. In the past this could be achieved by using two individual photoconductive cells with a beam-splitter arrangement or by using two or more separate cells with two or more separate amplifiers.

It is the principal object of this invention to provide a photoconductive cell having a large photoconductive area for searching and a small photoconductive area for tracking This combination photoconductive cell can be readily used with a single amplifier by alternatively using one of the photoconductive areas as a load resistor when the signal is on the other photoconductive area.

The photoconductive cell in accordance with this invention comprises two plates on which photoconductive material is deposited. The first plate has a photoconductive coating deposited thereon, leaving a central window with no photoconductive coating. The second plate has a photoconductive coating which covers an area corresponding to that of the window. The two plates are assembled together so that the photoconductive coatings are face to face, and the photoconductive coatings are separated by a thin layer of insulating material which is sandwiched between the two plates.

Other and incidental objects of this invention will be apparent from a reading of the following detailed description and an inspection of the accompanying drawing in which:

FIGURE 1 is a bottom view of the first plate showing the photoconductive coating thereon;

FIGURE 2 is a top view of the second plate showing the photoconductive coating thereon;

FIGURE 3 is a cross-sectional view taken along the lines AA of FIGURES 1 and 2 of the assembled photoconductive cell in accordance with this invention; and

FIGURE 4 is an electrical diagram showing how the photoconductive cell of this invention may be connected to an amplifier.

Referring now to FIGURE 1 there is shown a first plate of electrically insulating material 11 on which there is applied a photoconductive coating 13. The photoconductive coating 13 is provided with two electrodes 15 and 17 which may be thin layers of gold deposited on top of the photoconductive coating 13. The photoconductive coating 13 is provided with a centrally disposed window 19 on which there is no photoconductive coating. Adjacent to the window 19 and parallel to the electrodes 15 and 17 are two grids 20 and 21 which help to achieve constant field conditions on both sides of the window 19. The grids can be of gold and have a width of one mil.

Referring now to FIGURE 2 we see a second plate 22 on which there is deposited a photoconductive coating 23 which covers an area corresponding to that of the window 19 in the first plate 11. The second plate 22 is provided with two electrodes 25 and 27 which are deposited on top of the photoconductive coating 23 which extends under both electrodes 25 and 27.

FIGURE 3 is a cross-sectional view taken along the lines A-A of both FIGURES l and 2. It shows how the photoconductive cell of the invention is assembled. It can be seen that the photoconductive coatings 13 and 25 are on the inner surfaces of the two plates 11 and 22. The photoconductive coatings 13 and 23 are separated by a thin transparent layer 29 of insulating material such as quartz (or a cement transparent to infrared radiation) which is sandwiched between the photoconductive coatings 13 and 23.

FIGURE 4 shows how the photoconductive cell of the invention may be connected to an amplifier and illustrates its operation. The two photoconductive coatings 13 and 23, which are represented by resistors, are connected in series across a battery 31. When radiation impinges upon one of the photoconductive coatings, its resistance goes down, and this affects the potential at the junction 33 which is the input terminal of the amplifier 35. When a spot of light or radiation is focused on the photoconductive coating 13, the potential at the amplifier 33 is raised towards that of the positive terminal of the battery 31. ,When this spot of radiation is on the photoconductive coating 23 the potential at the junction 33 is lowered towards that of the negative terminal of the battery 31. There is thus a signal which indicates that the source of radiation is within the searching area of the cell and another signal which shows that the source of radiation is being accurately tracked by the tracking portion of the photoconductive cell.

I claim:

1. A photoconductive cell comprising two plates of electrically insulating material, said plates facing each other, a first photoconductive coating applied to the inner surface of one of said plates so as to leave therein a centrally disposed window on which there is no photoconductive coating, a second photoconductive coating applied to the inner surface of the other of said plates so as to cover an area corresponding to that of said window, a thin transparent layer of insulating material sandwiched between said first and second photoconductive coatings, and means to make electrical connections to both of said photoconductive coatings.

2. A photoconductive cell comprising two plates of electrically insulating material, said plates facing each other, a first photoconductive coating applied to the inner surface of one of said plates so as to leave therein a centrally disposed window on which there is no photoconductive coating, a pair of conductive grids adjacent to said window, a second photoconductive coating applied to the inner surface of the other of said plates so as to cover an area corresponding to that of said window, a thin transparent layer of insulating material sandwiched between said first and second photoconductive coatings, and means to make electrical connections to both of said photoconductive coatings.

References Cited in the file of this patent UNITED STATES PATENTS 2,001,672 Carpenter May 14, 1935 2,838,876 Smith June 17, 1958 

1. A PHOTOCONDUCTIVE CELL COMPRISING TWO PLATES OF ELECTRICALLY INSULATING MATERIAL, SAID PLATES FACING EACH OTHER, A FIRST PHOTOCONDUCTIVE COATING APPLIED TO THE INNER SURFACE OF ONE OF SAID PLATES SO AS TO LEAVE THEREIN A CENTRALLY DISPOSED WINDOW ON WHICH THREE IS NO PHOTOCONDUCTIVE COATING, A SECOND PHOTOCONDUCTIVE COATING APPLIED TO THE INNER SURFACE OF THE OTHER OF SAID PLATES SO AS TO COVER AN AREA CORRESPONDING TO THAT OF SAID WINDOW, A THIN TRANSPARENT LAYER OF INSULATING MATERIAL SANDWICHED BETWEEN SAID FIRST AND SECOND PHOTOCONDUCTIVE COATINGS, AND MEANS TO MAKE ELECTRICAL CONNECTIONS TO BOTH OF SAID PHOTOCONDUTIVE COATINGS. 